Mud wall disintegrator



Patented 11, 1941 UNITED STATES PATENT OFFICE v I uunwatzfi izmmnsronVernon 13.710110, venous, cam; assignm- Tidewater Associated OilCompany, New York,

N. Y., a corporation of Delaware No Drawing. Application l'ebruary 1c,1939,

Serial No. zsazar a claim (Cl. 166-21) m suitable power source about itslongitudinal axis.

The bit is provided with teeth of various shapes designed to bite therockaway in small pieces, and is lubricated by a continuous stream ofmud pumped down through the hollow'pipe and bit.

15 The stream returns'to a surface-sump, carrying the rock particles orcuttings with it, by pass! ing upwardly in the bore on the outside ofthe pipe. The mud is used to carry abrasive material in suspension andto cool the bit. 'The mud 001-.

2'0 umn also controls the oil and gas pressures encountered, and hasother important functions.

As the drill is driven deeper into the earth, a

. wallof partially dried mud is gradually built up around the outside ofthe bore.

25 countered, especially in deep wells, may cause a filtering or "bakingaction, and it is frequently foundthat the mud wall becomes so hard andimp rvious that it. must be removed before the oil can iiow into thebore. This removal'is hence to very important in determining the sizeand output of the well.

Qne of the common methods of removal involves treating the wall with anacid which will cause it wholly or partially 'to decompose, permit- 3oting the mud to slough off and be carried away in suspension by thefluid in the hole. Four of the factors governing the type of acid whichis suitable for'such use are: the rate of disintegrationofithe wall;safety inhandling; chemical I 4'0 inertness toward steel casing: andcost. Hydrochloric (muriatic) acid has been widely used.

It is relatively inexpensive," and reacts rapidly with the mud, but isdiiiicult to handle in the concentration necessary for treatment andtends to 48 attack the casing and drill pipe in the hole unlessinhibited;

Inhibiting the acid consists in adding thereto a substance which willreduce as far as possible the reaction of the acidwith the casing, whileat the '50 same time not interfering with the break-down of the mud.Such a substance could be a salt of any metal lying below=hydrosen inthe'electromotive series. One of the best and cheapest inhibitors iscuprous chloride. Others are described as in United StatesPatent1,877,504 to crane and The heat env -Sanford, which claims the use asinhibiting agents Q -of arsenic acid, arsenic tri-oxlde, and othersoluble arsenates and arsenites, as well-as a variety of othersubstances. Such inhibitors, however, are at best a compromise. Evenwith their use, it is necessary to introduce the inhibited acid to 5 thebore in undesirable concentrations. As the reaction progresses, the acidbecomes continually weaker; while a comparatively rapid reaction mayoccur at first, it tapers oif rapidly;- The time required for atreatment is usually from five to eighteen hours, but is dependent ona-number of factors, including the wall thickness, the depth of hole,and others.

My invention overcomes the diiiiculty esperil enced with even the mostsuccessful or the previously known methods by the use of a weak acid,together with a substance which will combine therewith in the bore insuch a way as to produce continually a freshsupply of a strong acid.The" concentrations thus secured will break down the mud wall rapidlywithout deleterious eifects on the casing. In pursuance of this method,,I preferably introduce in solution. acetic acid and magnesium chloride,which will react in the bore to form an inert salt and hydrochloric acidThe fundamental reaction in the chemical breakdown of a mud sheath isthat between the acid and calcium carbonate, which is an importantconstituent of nearly all native clays. s The calcium carbonate isdispersed throughout the mud; it combines with acid to form an unstablecompound, carbonic acid, which again breaks down intowater and carbondioxide gas. The mudbecomes honeycombed by the decomposition of thecalcium carbonate, so:that the remainder of the wall is easilypenetrated by and sloughs oil with the fluid circulating or standing inthe hole.

It will be seen from the above description that M the primary object ofmy invention is to provide more eifective means for removing the mudsheath from'well bores. Other; objects are: to

. provide a continuously replenished supply of acid sheath dlsintegratorwhich may be handled without danger by operators; and to provide adisintegrating agent in more economical form.

My invention possesses numerous other objects and features of advantage,some of which. together with the foregoing, will be set forth in thefollowing description. It is, therefore, to be understood that myinvention is applicable to other substances, and that I do not limitmyself, in any way, to the exact showing 0! the present application, asI may adopt various other apparatus embodiments, utilizing the method,within thescope of the appended claims.

Removal is accomplished by pumping the solution of my invention into thebore, either directly or through the drill stem. The solution is allowedto remain in the hole for about ten hours. This time is, of course, avariable, depending on the percentage of carbonatesin the wall, thepermeability and the thickness, the temperature in the hole, and thepressure head due to the overlying fluid column. .Where ordinary acidtreatment is used, the usual range of times is from five to eighteenhours, so it will be seen that my method compares very favorably withthe prior art in this respect.

If the wall is to be removed along only a portion of the bore, thesolution is spotted" in place by displacing the mud or liquid in thatportion of the hole.

My departure from the previous art lies in the use of acetic acidinstead of hydrochloric acid, and the addition of substances theretowhich will increase the reactive rate with the mud. I prefer to usemagnesium chloride for this pur-- pose, the most eflfectlve proportionsbeing 4.0%

magnesium chloride and 25% of glacial acetic acid, by weight.

A milld reaction occurs when the solution is mixed, due to the smallproportion of magnesium. chloride present. Some magnesium acetate andhydrochloric acid is formed, leaving surplus I acetic acid. Using theabbreviation Ac to represent the acetate radical (CHaCOO), this initialreaction may be written:

The mixed solution represented by the rig-ht half of the Equation 1 thenbegins to react with the mud in the formation. The hydrochloric acidreacts with the calcium carbonateto form calcium chloride and carbonicacid. The latter is unstable and breaks down into water and carbondioxide;- the former provides a source of the chloride radical. forsubsequent reaction with one excess acetic acid. At the same time, theacetic acid reacts with the calcium carbonate to form calcium acetateand carbonic acid. The latter breaks down into water and carbon dioxide,and the former remains inert as did the magnesium acetate previouslyiormed. These reactions may be written:

The surplus acetic acid remaining is still available for reaction withthe mud directly, as in Equation 3, and through intermediate combinationwith the calcium chloride oi Equation 2:

The hydrochloric acid produced as in Equation 4 will in turn react withthe mud as in: Equation 2, the process continuing until all the aceticacid is-spent.

The continual replenishment of the hydro-- \chloric acid as it isconsumed makes possible a comparatively rapid breakdown of the mud wallwithout there being present at any time a corrosive concentration, whilethe reactive power is much greater than if dilute hydrochloric alone wasused.

The proportions suggested above have been found to be the mosteconomical for a positive reaction. An increase in the amounts ofglacial acetic acid and magnesium chloride present has little effect onthe rate of reaction, while a decrease causes a marked decrease in therate of disintegration.

I have found that the breakdown may be speeded by the addition to thesolvent of a wetting agent,.which will reduce the surface tension of thesolution and permit it to penetrate the mud wall more freely. One suchwetting agent which is satisfactory is known by the trade-mark "AerosolOT", the composition of which is not known exactly to me, although it isbelieved to be an ester of a sulfonated bi-carboxylic acid. The properproportion is 1 to 2 parts of the commercial 10% strength solution to1,000 parts of solvent, by weight, or 1 to 2 parts of full strengthAerosol CT" to 10,000 parts of mud solvent, by weight.

Another means of increasing the completeness of the reaction is to addmore calcium carbonate to the drilling mud. Experience in the fieldindicates, however, that the method is entirely satisfactory withVentura New Mud, "Ventura Reweak alkali and a strong acid could be used,subject to the further limitations, that it be soluble in the initialsolution, and that all the reaction products be similarly soluble.Chlorides are preferred. because the hydrochloric acid is the mostactive. The limitation as to solubility eliminates all the other alkalisbut strontium. Strontium chloride could be substituted for the magnesiumchloride, in the same proportions. Its cost, however, is greater,rendering it less economical to use at present.

It is also apparent that other weak acids might be substituted foracetic acid, provided that the same requirements as to solubilityspecified above, are met. Such a substitute would be readily ,reactivewith the mud, inert toward the steel pipe and tools, be low in cost, andreact with the magnesium chloride to liberate hydrochloric acid Ithrough the reaction period.

The safety in handling the solution I have described may be judged fromthe fact that it may be placed in the mouth'for a while with no un-,

. changes makes it possible for the remaining mud.

to slough ed and be carried in suspension by the the usual manner.

It will be apparent to those skilled in the art that I have invented adisintegrating solution which meets the requirements as to speed,safety, inertness toward tools and casing, and low cost; It provides anon-corrosive solution which will react in the bore to provide acontinuous supply of acid reacting rapidly with the bore, and a methodof treatment which eliminates the industrial hazard always present inhandling very strong acids.

What I claim is:

1. The method of disintegrating the mud sheath within a well bore whichcomprises treating the mud with a solution including, by weight,substantially 25% of glacial acetic acid, and 4.0% of magnesiumchloride.

2. The method of disintegrating the mud sheath within a well bore which.comprises injecting a solution including, byweight, substanand combiningin situ with said salt to release an acid strongly reacting withconstituents of said sheath, permitting the'solution to remain thereinfor about ten hours, and removing the resultant solution from the bore;

4. A method or treating a mud wall in a bore having a casing therein,which comprises pumping therein a solution having 25% by weight. of anacid not appreciably reactive with said casing and 4% by weight of asoluble salt of a weak alkali and a strong acid, said non-reactive acidand said soluble salt being reactive to form an inert soluble salt andan acid strongly reactive with said mud wall, permitting the reactionsto continue for about ten hours, and pumping out the end productsresulting from said reactions.

5. The mud wall disintegrating treatment which comprises placing incontact therewith a solution comprising one to two parts in ten thousandof a wetting agent, four per cent of a soluble reactive with said mud,assisting said highly reactive acid to penetrate said mud thoroughly,maintaining contact between said mud and said solution for about tenhours, and pumping the resultant products out of said hole.

6. A mud wall disintegrating solution, comprising a soluble salt formedfrom a weak alkali and a strong acid, and an acid relatively inert withrespect to steel but able to combine with said salt adjacent said wallto form a strong acidand other soluble salts.

7. A mud wall disintegrating solution, com

